Add go-llm v2: redesigned API for simpler LLM abstraction

v2 is a new Go module (v2/) with a dramatically simpler API:
- Unified Message type (no more Input marker interface)
- Define[T] for ergonomic tool creation with standard context.Context
- Chat session with automatic tool-call loop (agent loop)
- Streaming via pull-based StreamReader
- MCP one-call connect (MCPStdioServer, MCPHTTPServer, MCPSSEServer)
- Middleware support (logging, retry, timeout, usage tracking)
- Decoupled JSON Schema (map[string]any, no provider coupling)
- Sample tools: WebSearch, Browser, Exec, ReadFile, WriteFile, HTTP
- Providers: OpenAI, Anthropic, Google (all with streaming)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/response.go

@@ -0,0 +1,34 @@
+package llm
+
+// Response represents the result of a completion request.
+type Response struct {
+	// Text is the assistant's text content. Empty if only tool calls.
+	Text string
+
+	// ToolCalls contains any tool invocations the assistant requested.
+	ToolCalls []ToolCall
+
+	// Usage contains token usage information (if available from provider).
+	Usage *Usage
+
+	// message is the full assistant message for this response.
+	message Message
+}
+
+// Message returns the full assistant Message for this response,
+// suitable for appending to the conversation history.
+func (r Response) Message() Message {
+	return r.message
+}
+
+// HasToolCalls returns true if the response contains tool call requests.
+func (r Response) HasToolCalls() bool {
+	return len(r.ToolCalls) > 0
+}
+
+// Usage captures token consumption.
+type Usage struct {
+	InputTokens  int
+	OutputTokens int
+	TotalTokens  int
+}